1. Field of the Invention
The present invention relates to a control apparatus for an electric vehicle which includes a brake control device for stabilizing a vehicle behavior by controlling a friction brake of a vehicle wheel.
2. Description of the Related Art
In an electric vehicle including an electric motor for driving a vehicle wheel, the electric motor is operated as a power generator during vehicle braking such that the vehicle is braked using a regenerative torque generated by the electric motor. However, a braking force obtained by a regenerative brake employing an electric motor is affected by a battery characteristic and so on, and therefore a braking force of a required magnitude is typically secured with stability by employing a friction brake such as a disk brake alongside the regenerative brake. Further, when a regenerative brake and a friction brake are used in combination, it is important to coordinate the regenerative brake and the friction brake (see Japanese Patent Application Publication (JPA) No. 1999-4503 and Japanese Patent Application Publication No. 1998-297462, for example).
Incidentally, as described in JPA No. 1998-297462, an electric vehicle is also installed with a brake control device such as an antilock braking system (ABS). The brake control device estimates a vehicle behavior based on signals from a vehicle wheel speed sensor, an acceleration sensor, and so on, and stabilizes the vehicle behavior by operating the friction brake to control the braking force applied to each vehicle wheel. Furthermore, when the other brake control device such as the ABS is operative, torque control is executed on the electric motor simultaneously by a motor control unit. By controlling the electric motor in coordination with the brake control device in this manner, the vehicle behavior is further stabilized.
However, when an abnormality occurs in a control system such as the vehicle wheel speed sensor or a communication network, the motor control unit cannot learn the operational state of the brake control device, and therefore the brake control device and the electric motor cannot be coordinated easily. For example, when the coordinated control of the brake control device and the electric motor becomes difficult, a regenerative torque may be output from the electric motor even though ABS control is executed. When the electric motor is controlled in this manner, the vehicle wheels may lock, causing instability in the vehicle behavior, and as a result; a steering stability of the electric vehicle may deteriorate.